1. Technical Field
The present invention relates to spark plugs for internal combustion engines which have an annular ground electrode arranged so as to face an outer circumferential surface of a center electrode, and to methods of manufacturing the spark plugs.
2. Description of the Related Art
Japanese Patent No. 5075127 discloses a spark plug for an internal combustion engine of a motor vehicle or a cogeneration system. The spark plug has an annular ground electrode arranged so as to face an outer circumferential surface of a center electrode. The ground electrode is fixed to a housing (or metal shell) by a crimped portion of the housing; the crimped portion is crimped at a distal end of the housing against an outer periphery of the ground electrode. Between the outer circumferential surface of the center electrode and an inner circumferential surface of the annular ground electrode, there is formed an annular spark gap.
However, with the above configuration, the ground electrode is in contact with the housing on the outer periphery of the ground electrode. Therefore, the heat dissipation path from the inner circumferential surface of the ground electrode, which faces the spark gap, to the housing is long, causing the temperature of the ground electrode to be easily increased. Further, with increase in the temperature of the ground electrode, the amount of wear of the ground electrode at the spark gap is increased, thereby accelerating increase in the radial width of the spark gap. Consequently, the time needed for the radial width of the spark gap to reach an upper limit is shortened; when the radial width of the spark gap is above the upper limit, the spark plug cannot normally function. As a result, it is difficult to secure a long service life of the spark plug.
Moreover, with the above configuration, the ground electrode is disposed inside the housing and the spark gap is formed on the proximal side of the distal end of the housing. Therefore, there is a problem that it is difficult for the flame produced by a spark discharge in the spark gap to grow. That is, there is a problem that the flame makes contact with the housing and thus loses heat to the housing. Consequently, it is difficult to secure a high ignition capability of the spark plug (i.e., a high capability of the spark plug to ignite an air-fuel mixture in a combustion chamber of the engine).
Furthermore, with the above configuration, since the ground electrode is fixed inside the housing by the crimped portion of the housing, it is difficult to adjust the relative position of the ground electrode to the center electrode and thus difficult to adjust the spark gap.
More specifically, to accurately form the spark gap between the outer circumferential surface of the center electrode and the inner circumferential surface of the ground electrode, it is necessary to accurately position the ground electrode with respect to the center electrode. However, due to dimensional and assembly variations in the components of the spark plug (e.g., the housing), it is impossible to accurately form the spark gap only by accurately arranging the ground electrode at a predetermined position with respect to the housing. Therefore, it is necessary to adjust the relative position of the ground electrode to the center electrode. However, since the ground electrode is fixed inside the housing by the crimped portion, the ground electrode is restricted from being moved in a radial direction of the spark plug, thereby making it difficult to adjust the spark gap.